The presence of large deposits of oil shale in the high plateau, semi-arid region of the western United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is, in fact, a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen" which, upon heating, decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the ground surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents such as U.S. Pat. Nos. 3,661,423; 4,043,597; 4,043,598; and 4,192,554; and in U.S. patent application Ser. No. 070,319 (now abandoned) filed Aug. 27, 1979, by Chang Yul Cha, entitled TWO-LEVEL, HORIZONTAL FREE FACE MINING SYSTEM FOR IN SITU OIL SHALE RETORTS. Each of these applications and patents is assigned to Occidental Oil Shale, Inc., assignee of this application, and each is incorporated herein by this reference.
These patents and applications describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale, wherein such formation is explosively expanded to form a stationary fragmented permeable mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort, or merely as a retort. Retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method of supplying hot retorting gases used for converting kerogen contained in the oil shale as described in U.S. Pat. No. 3,661,423, includes establishing a combustion zone in the retort and introducing an oxygen-supplying retort inlet mixture into the retort to advance the combustion zone through the fragmented mass. In the combustion zone, oxygen from the retort inlet mixture is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas, and combusted oil shale. By the continued introduction of the retort inlet mixture into the fragmented mass, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the retort inlet mixture that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting". Such decomposition in the oil shale produces gaseous and liquid products, including gaseous and liquid hydrocarbons, and a residual carbonaceous material.
The liquid products and the gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, collect at the bottom of the retort and are withdrawn. An off-gas is also withdrawn from the bottom of the retort. Such off-gas can include carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process.
U.S. Pat. Nos. 4,043,597; 4,043,598; and 4,192,554 disclose methods for explosively expanding formation containing oil shale toward horizontal free faces to form a fragmented mass in an in situ oil shale retort. According to such a method, a plurality of vertically spaced apart voids of similar horizontal cross-section are initially excavated one above another within the retort site. A plurality of vertically spaced apart zones of unfragmented formation are temporarily left between the voids. A plurality of horizontally spaced apart vertical columnar explosive charges, i.e., an array of explosive charges, is placed in each of the unfragmented zones and detonated to explosively expand each unfragmented zone upwardly and/or downwardly toward the void or voids above and/or below it to form a fragmented mass having an average void volume about equal to the void volume of the initial voids. Retorting of the fragmented mass is then carried out to recover shale oil from the oil shale.
U.S. patent application Ser. No. 070,319 discloses a method for explosively expanding formation containing oil shale toward a horizontal free face to form a fragmented mass in an in situ oil shale retort. According to such a method, a void having a horizontal cross-section similar to the horizontal cross-section of the retort being formed is initially excavated. A plurality of vertically spaced apart zones of unfragmented formation are left above the void. Explosive is placed in each of the unfragmented zones and detonated for explosively expanding such zones toward the void to form a fragmented mass in the retort having an average void volume about equal to the void volume of the initial void. The overlying zones can be expanded toward the void in a single round or a plurality of rounds. Retorting of the fragmented mass is then carried out to recover shale oil from the oil shale.
It has been determined that the efficiency of in situ retorting of oil shale is enhanced when gas flow through all regions of the fragmented mass of formation particles in the retort is uniform.
To enhance uniform gas flow through vertical in situ oil shale retorts, techniques have been developed for forming a fragmented permeable mass in such a retort which has a reasonably uniform permeability in horizontal planes across the retort. This eliminates bypassing portions of the fragmented mass due to gas channeling and results in improved yields of shale oil from the retorting process.
However, when the permeability of a fragmented mass of formation particles is reasonably uniform in horizontal planes across a vertical retort, gas flow may still not be completely uniform. This is because it may not be practical or economical to provide enough retort fluid inlets and outlets at locations necessary to result in equal length fluid flow paths throughout the entire fragmented mass between such inlets and outlets. In other words, with a limited number of retort fluid inlets and outlets, it is necessary for retoring fluids to traverse various length paths if the entire fragmented permeable mass is to be effectively contacted by the fluids.
When the permeability of the fragmented mass is uniform, as described above, the gas velocity along each flow path is theoretically about inversely proportional to the length of the flow path. Thus, the velocity of gas flow along a shorter path between a retort inlet and outlet will be higher than the velocity of the gas flow along a longer path. It has been estimated theoretically and determined experimentally that the rate of flame front or combustion zone advance through a fragmented mass is proportional to the gas velocity. Therefore, a combustion zone advances more rapidly through a fragmented mass along shorter gas flow paths and less rapidly along gas flow paths of greater length.
Therefore, when all gas flow paths through a fragmented mass are not equal and the permeability is uniform, the combustion zone will not advance in a flat, planar wave, but will be skewed. This can decrease the efficiency of the retorting operation.
One attempt to solve the problem of uneven retorting fluid flow through a vertical retort is disclosed in U.S. Pat. No. 3,951,456 to Ridley. This patent discloses a vertical retort with one or more fluid inlets through its top and one or more fluid outlets at its bottom. If desired, the fluid outlets can be at the periphery of the retort. It is disclosed that the rubble pile in the retort has a variable bulk permeability which increases from the shortest to the longest flow path between a retorting fluid entrance and exit.
The rubble pile is retorted progressively from the top to the bottom, i.e., the combustion zone moves vertically through the retort.
Alternatively, however, it can be desirable in an in situ oil shale retort to introduce combustion supporting gas to the fragmented mass along one upper edge of the retort and withdraw gas from along the opposite lower edge. Thus, the direct gas flow path through the fragmented mass is skewed from vertical and extends generally diagonally across the retort. This is considered desirable for several reasons, one of which being that the difference between the longest and shortest gas flow paths through the fragmented mass is reduced compared, for instance, to the difference in the longest and shortest gas flow paths through a retort having one inlet at the center of the top and one outlet at the bottom.
There is a need for providing such a retort with enhanced uniformity of gas flow.